This invention relates to a process of pressure reaction injection molding (PIM), from liquid reactive resin-forming starting material, molded bodies of thermosets (or "duroplastics") having from none to a major amount of filler and from none to an effective amount of reinforcing material.
The invention further relates to apparatus for carrying out the above-mentioned novel process in practice.
It is known to use injection molding techniques for producing shaped bodies of thermosets (duroplastic resin materials), also known as duromers such as urea-formaldehyde resins (UF), melamine-formaldehyde resins (MF), phenol-formaldehyde resins (PF), epoxide resins (EP) as well as unsaturated polyester resins (UP) such as those produced from diallyl phthalate (DAP). Thermosets ("duroplastics") are known to be formed from so-called reactive, hardenable (thermosetting) resins by irreversible cross-linking. Reactive, thermosetting resins are by definition liquid or liquefiable resins or mixtures of substances which are capable of hardening by polymerisation, polycondensation or polyaddition, per se or after adding reactants such as hardeners, accelerators and the like substances. As a rule injection molding and compression molding of thermosets (duroplastics) are carried out with adducts of resin and hardener or pre-reacted resins as starting materials. Thereby, less energy is released in the form of heat which must be dissipated. Moreover, the formal product shows less shrinkage.
It is also known to produce integral foams from polyurethanes (PUR) by means of reactive injection molding. In this process, large amounts of a multi-component system are quickly dosified and mixed, and the reaction mixture is then injected into the internal chamber of an injection mold and is hardened under pressure by the released foaming gases to afford the molded body.
In lieu of reactive resins, in particular epoxide resins, there can also be processed according to the reactive liquid injection (transfer) molding process (LIM), their monomers or, at most, their oligomers. Oligomers are, by definition, low molecular reactive resins having a molar mass of 10.sup.2 to 10.sup.4. Monomers or oligomers always contain large amounts of admixed or dissolved air. It is, therefore, difficult to produce completely air-free molded bodies consisting of thermosets (duroplastics) which are pure or contain filler and/or reinforcing agent, without deaeration.
The molds used in the known injection molding techniques for instance when flat discs or plates, e.g. grammophone record bodies are to be produced, usually consist of two mold halves, enclosing a reaction space when joined together with a tight seal, and it is conventional to maintain both mold halves at the same temperature during the hardening step of the mass injected through an orifice in one mold half.
By using pivotably mounted mold carriers the molds can be brought into a position which is most favorable for filling as well as the removal of released air. Moreover the filling holes can be arranged and dimensioned in a manner such that no air will be occluded and that a suitable filling rate can be attained. Nevertheless, difficulties are likely to occur which require deaeration under vacuum.
Due to the shrinking of the resin during the hardening reaction which may amount to about 1 to 10% of the total volume of the molded body, faults such as sink marks (cavedin areas or other shrinkage marks) may occur in the surface of the molded body. During the process of reactive injection transfer molding of formulated thermosets, it is, as a rule, not possible to eliminate these effects satisfactorily, merely by formation of adducts of resin and hardener or by prepolymerization of the resin, because such adducts or prepolymerizates have at room temperature higher viscosities or may even be solid, which fact renders turbulent mixing very difficult. Moreover the costs of raw materials are thereby increased.
A further drawback of the known process resides in a very considerable rise of the temperatures of the mass due to the exothermic nature of the hardening reaction; this may even lead to a decomposition of the molded body, unless relatively low starting temperatures are used which in turn leads to long reaction times
In practice, there are mainly produced molded bodies containing filler or reinforced by glass fibers, and less frequently molded bodies which are free from fillers and reinforcing materials. Filler-containing molded bodies may contain up to 60 and more percent by weight of filler. The fillers cover up many deficiencies and have a favorable influence on the enthalpy of the hardening reaction and on shrinkage as they do not themselves participate in the hardening reaction. Glass fiber-reinforced molded bodies have much in common with molded bodies which are free from fillers and reinforcing materials, because they are transparent and any faults in their surface or in the interior of the molded bodies can be immediately detected. Also, such molded bodies free from fillers and reinforcing agents or molded bodies which are reinforced by glass fibers are particularly suitable for judging the quality of a molding technique.
In a known process, elongated bodies of synthetic resin material are produced in an upright mold in which the hardening temperature is applied in a region which is gradually moved from the bottom end upward, while fresh hardenable mass is added via the open top end of the mold. This process is not applicable to flat bodies such as plates or discs, e.g. grammophone records; in the latter case one flat side of the disc must be given very fine elevations and depressions in its surface which would be distorted or destroyed by the successively added mass.
It has now been found that other difficulties occurring when thermosets are produced in injection molding and, in particular, in the reactive liquid injection molding process, are due to the fact that hardening of the resin begins at the hot surface of the molded body, so that an outer hull of the molded body is already hardened while unhardened resin is still present at its core. When the resin in the interior of the molded body is hardened it will shrink, but, as the surface is already solidified, no further resin can reach the interior, and sink marks (caved-in areas of the molded body and other shrinkage marks) are the result.